There are many situations when it would be desirable to selectively activate multiple downhole devices. It is also preferable that one can do so not necessarily following a particular pre-determined sequence.
Typically there are multiple casing valves, each with a sliding sleeve and cemented in a well. Due to the heterogeneous nature of formation, one might not want to open all the valves simultaneously so that the fracturing operations can be performed separately for different layers of formations. A known method of selective actuation is to use different diameter balls to open the valves from bottom up. For example, each valve can have a restriction that will be blocked, or engaged, by a ball having a diameter at least as large as the restriction. The restrictions are then ordered so that the narrowest constriction is at the bottom, and the restriction size gradually is increased from the bottom up. A ball with smallest size is first dropped into the well. The size of the ball is designed so that it will go through all the valves except the bottom valve. The ball will be stopped by the bottom valve so that the sliding sleeve of the bottom valve will be pushed to the “open” position and expose the wellbore to cemented formation. Then the fracturing operation through the valve N can be executed. After that, the next size of ball will be dropped to activate the N−1 valve.
While the graduated ball technique works, it has an inherent limitation based on how many stages can be implemented in a string of valves downhole. Since the order of opening of the valves depends on the sizes of balls dropped, the radius of restriction in each stage needs to be different. Additionally, for a given size of the completion, the number of different sizes (radii) of restrictions is finite and is often less than desirable. Typically, a maximum of eight balls can be dropped, and this can severely impact the effectiveness of the multistage fracturing operation. For example, it is not uncommon to make wells economical that have 20 or more production zones.
Another known technique utilizes control lines between adjacent zones to activate restrictions. Once a restriction in a particular valve is activated, it is ready to catch a dart dropped from the surface in order to open this particular valve. However, there is a possibility for damage of the control lines during run-in-hole, especially in horizontal wells. A damaged control line might result in only those zones below the damaged zone being capable of production. Another drawback of this technique is that as the thickness of the valve increases, the internal diameter of the valve decreases in order to accommodate the complex hydraulic mechanisms in the valve.
Yet another known technique is described in commonly-owned U.S. Pat. Publ. No. 2015/0000935. This technique uses darts having moveable pieces that allow the dart to radially compress when engaging restrictions. However, since the darts have moveable pieces, during operation various portions of each dart piece are put under stresses and strains. As a result, a careful selection of material or materials should be made for the dart pieces.
Yet another known technique is described in commonly-owned U.S. Pat. Publ. No. 2014/0299319. This technique uses casing segments having a plurality of casing ribs. A sleeve member includes rocker members that are engaged by darts pumped through the well. Each dart that engages the rocking members causes the sleeve to move one rib position.